Veterans from Afghanistan and Iraq (OEF/OIF) are at high risk for becoming overweight and obese; 86% were overweight or obese at their first visit to the VA as reported in one recent study. However, existing VA programs are not designed for younger Veterans who are more comfortable with technology-mediated interventions than older Veterans and who may not yet have developed obesity related chronic diseases. Additionally, OEF/OIF Veterans include a relatively high percentage of women compared to previous Veteran cohorts. These differences should be addressed in lifestyle interventions customized to OEF/OIF Veterans. Technology-mediated lifestyle interventions that include continuous, objective home monitoring of physical activity, automated internet mediated feedback, and e-coaching increase physical activity and improve weight loss in non-Veteran populations. When delivered on a large scale, such interventions represent low cost but effective alternatives to face-to-face lifestyle change interventions. The VA health care system is in a strong position to implement such interventions on a national scale because of existing structures such as a national electronic medical records system. Such interventions can be centrally administered and marketed directly to Veterans, capitalizing on economies of scale, expanding intervention reach, and reducing the burdening of recruitment on the existing health care team. However, with the exception of one pilot study, prevention focused technology-mediated physical activity programs that include continuous, objective home monitoring of physical activity, automated internet or cell-phone mediated feedback, and e-coaching have not been customized and tested for OEF/IOF Veterans. This project will test the feasibility and effectiveness of the prevention focused, internet mediated healthy lifestyle Stay Strong program tailored to the needs, preferences and demographics of OEF/OIF Veterans. If successful, the Stay Strong program could be implemented as a national program to augment the VA's current panel of options for OEF/OIF Veterans who need support to maintain a healthy lifestyle and prevent future disease. The specific aims of this project are t 1) evaluate the impact of an automated, centrally administered, internet-mediated, physical activity intervention, Stay Strong, on physical activity among OEF/OIF Veterans; 2) evaluate the impact of Stay Strong on the secondary outcomes of weight loss, depression and pain among OEF/OIF Veterans; and 3) test for moderation of the intervention effect of the Stay Strong intervention by gender with respect to the primary outcome of minutes of moderate to vigorous physical activity per day, as well as secondary outcomes of weight loss, depression and pain. In this randomized, waitlist controlled study, OEF/OIF Veterans will be randomized into either a waitlist control arm or the Stay Strong intervention arm for one year. The primary outcome is change in minutes of moderate to vigorous physical activity per day averaged over 7 days. Weight loss, pain and depression are secondary outcomes. Because gender moderates the impact of physical activity interventions, we will tailor StayStrong on gender and over-sample women. The trial is innovative in that study staff will have no face-to-face contact with participants. All participant recruitment, eligibility screening, informed consent, baseline assessment, randomization, intervention delivery and outcome assessment will be internet or cell-phone mediated. A constrained longitudinal data model in which baseline physical activity is modeled as a dependent variable in conjunction with the constraint of a common baseline mean across the treatment group will test for a between- group comparison in physical activity from baseline to 12 months in the intervention and waitlist groups.